1. Field of the Invention
The invention relates to methods and apparatus for maintaining rail spacing and minimizing rail curvature in electromagnetic launchers (railguns).
2. Design Considerations for Mobile Railguns
Railguns use electromagnetic force to accelerate a projectile which completes an electrical path incorporating two or more substantially parallel conducting rails. Peak electrical currents exceed 3,000,000 amperes (3.0 MA) in certain guns, and the resulting magnetic pressure may result in a total (radial) force acting to separate the rails in excess of 90 kips/in. Actual movement of the rails, however, should not be so great that electrical contact is lost or substantially reduced between either rail and the projectile.
Should rail-projectile contact be lost, the resulting plasma contact would generally have a higher associated voltage drop than a metal-to-metal contact, thus reducing current flow and the amount of magnetic force applied to the projectile. Thus in stationary railguns, high-strength (and relatively heavy) structural members are employed to provide the substantial restraining forces needed to prevent rail separation. The required structural mass may be reduced somewhat through the use of preloaded elements (e.g., ceramic rail supports heavily preloaded by an outer steel shell), but the resulting railgun barrel is still too heavy for mobile applications. Further weight reductions require replacement of ceramic rail supports with members having lower preload requirements, and replacement of a steel outer shell with a lighter preloading member.
Weight reductions in structural elements must not, however, compromise the stiffness required to keep the railgun barrel straight within narrow tolerances. This is because the muzzle velocity of railgun projectiles may range from two to ten kilometers per second. Thus, the rails and their supporting structures comprising the railgun barrel must have the requisite stiffness to avoid droop or sag which would impair performance by deviating the projectile path from a straight line. If the railgun is to be mobile, the barrel must also be sufficiently light and stiff to allow the use of conventional recoil and aiming mechanisms (i.e., the barrel should be stiff enough to withstand abrupt changes in longitudinal acceleration and high slew rates).
Another aspect of railgun design is maximization of muzzle energy, which for a given rail current is approximately linearly related to the inductance per unit length (L') of the rails within a railgun barrel. L'tends to be reduced by the presence of metal rail supports or any other electrical conductor near the rails if substantial eddy currents can be induced parallel to the longitudinal axis of the barrel. Ceramic rail supports are thus theoretically superior to metal supports because they are electrical insulators, but the large preload requirement needed to prevent cracking of the insulators is not compatible with weight constraints in a mobile railgun.
As a substitute for ceramic rail supports, stacked iron or steel laminations have been used to fore railgun barrels and to provide needed radial support for the rails. Laminated metal rail supports have less longitudinal stiffness than solid metal, but eddy current losses in the laminations are substantially smaller than such losses would be in comparable solid metal supports if the laminations are sufficiently thin and electrically insulated from one another.
To increase longitudinal stiffness, stacked laminations are bonded together (preferably by the insulating material separating them from each other and from the rails). Without additional support, however, bonded laminations have limited resistance to transverse forces (e.g., due to gravity or barrel inertial forces) which tend to distort the railgun barrel along its longitudinal axis (which substantially parallels or is coincident with the direction of projectile travel). Hence, laminations have heretofore been used in railguns having fixed mounts where longitudinal railgun barrel stiffness could be provided by additional (fixed) structural systems (e.g., longitudinal bolts). No design for railgun barrels comprising stacked laminations and having the strength, stiffness and lightness required for mobile applications has been demonstrated or proposed.